1. Field of the Invention
This invention relates to a semiconductor device having semiconductor elements and at least one fuse sealed by means of resin or in an airtight fashion, and more particularly, to a power semiconductor device in which the main current in the semiconductor element is large and which necessitates a reliable fuse function.
2. Description of the Related Art
A three phase full-wave semiconductor rectifier device [Japanese Patent Disclosure (Kokai) No. 61-77353], which is used in cars and has six semiconductor rectifier elements integrally sealed by means of resin, is an example of the prior art semiconductor device having no fuse function, and is explained with reference to the accompanying drawings.
As is shown in FIG. 28, insulation layer 5 is formed on radiation plate 4, with conductive patterns 6a and 6b being respectively formed on layer 5 and plate 4. Disk portions of diode 7 are soldered on conductive patterns 6a and 6b. Casing 11 is bonded on radiation plate 4, so as to surround diode 7, input terminal 10, and B.sup.+ terminal 9. Further, epoxy resin 15 fills casing 11 as well as the space which is provided inside the terminal holder (12) and over the casing (11).
FIG. 29 is a circuit diagram of a three phase full-wave rectifier device, the device being represented by a portion surrounded by broken lines; 7a to 7f denotes diodes; 9a and 9b, external output terminals; 10a to 10c, internal input terminals; and 8, a capacitor. In addition 14 indicates a stator coil of the three phase A.C. generator; and 16, a battery.
When diodes 7a and 7b; 7c and 7d; or 7e and 7f of the above-described prior art semiconductor rectifier device having no fuse function are simultaneously deteriorated to lose the rectifier function and set in the short-circuited condition due to an external effect such as an overvoltage, overcurrent, or mechanical shock applied to the device, or as a result of an internal effect such as deterioration of the element, then an abnormal current will flow in the device, and generate an excessive heat.
A rectifier as disclosed in Japanese Utility Model Publication No. 60-23985 will now be described as a second example of the prior art semiconductor device, in this case, one having the fuse function.
As is shown in FIG. 30, rectifier terminal 22 and connection terminal 23 are arranged a specified distance apart from each other in resin casing 21. Silicon diode chip 24 is soldered on the end portion of rectifier terminal 22, and fuse wire 25 is bonded between silicon diode chip 24 and connection terminal 23, to constitute a current path, and the casing is filled with resin 26. Thus, fuse wire 25 is contained in the rectifier, and, when an overcurrent flows therethrough, is blown so as to protect the rectifier and other associated parts.
Since a rectifier having the fuse function uses a fuse wire as a fuse element and the fuse wire is buried in the epoxy resin, it will be difficult to attain a reliable fuse characteristic. More specifically, when an abnormal current flows, causing the fuse to blow, the epoxy resin changes in quality (it often becomes carbonized). As a result, a leak current can flow, via the quality-changed layer, and the fuse function cannot be sufficiently effected even if the fuse is blown out. In addition, that portion of the fuse wire which lies near the bonding portion of the fuse wire and the terminal may be easily subject to variation in thickness at the time of bonding, thus causing the value of a blowout current to be changed. Since the material used at the bonding portion is a fuse, the bonding strength becomes weak. Further, it is difficult to simply apply the fuse structure of the rectifier to an ordinary semiconductor device.
As described above, in a semiconductor device having no fuse function, an abnormal current can flow and abnormally high heat will be generated in the device or in the peripheral circuits, due to possible abnormal effect. In particular, in a module formed in a package or a semiconductor device dealing with a relatively large current, it is important to provide the fuse function in the device.
The fuse function is provided in a circuit (device or the associated circuit) so that the circuit is protected from damage by blowing out the fuse by Joule heat generated therein, when a current larger than a preset value flows in the fuse for a predetermined time.
In the prior art semiconductor device having the fuse function, a simple fuse wire is used, and it is difficult to always attain a reliable fuse function as will be described later. For example, it is difficult to always attain a constant blowout current, a constant time from the starting of the flow of abnormal current to the blowing-out of the fuse, a perfect blowout without causing leak current, and the like.